Download Gimp. It's free, just do a google search.
Then go to www.planetrenders.net and make an account. You can get a ton of pics in .png format. .png basically makes everything but the point of interest transparent. Then it's as simple as finding or making a background you like and copying and pasting, and then re-sizing.
It can get get much more complicated than that, but that's what you need to know to get started.